The present invention relates to a method for aligning first and second objects, relative to each other, and an apparatus for practicing this method and, more particularly, to a method and apparatus for aligning a mask and a wafer relative to each other when an image of a circuit pattern is transferred to a wafer in the steps of manufacture of a semiconductor.
In the steps of manufacturing a semiconductor device such as a VLSI, a circuit pattern is normally exposed on a wafer by an exposure apparatus. In this apparatus, when a circuit pattern preformed on a mask is irradiated with X-rays, an image of the circuit pattern is transferred to a wafer. Before the circuit pattern is transferred, the mask and the wafer must be accurately aligned relative to each other. More specifically, the mask and the wafer are arranged to face each other. However, the mask and the wafer must be aligned in a direction along opposing surfaces thereof.
One method for aligning a mask and a wafer uses a double diffraction grating. More specifically, one-dimensional diffraction gratings are respectively formed on the mask and the wafer. The two diffraction gratings have the same pitch, and their stripes (bars) are oriented in the same direction. When a laser beam is radiated onto the upper surface of the mask, light beam diffracted by and transmitted through the diffraction grating of the mask, is diffracted and reflected by the diffraction grating of the wafer, and is again diffracted by the diffraction grating of the mask. Diffracted light beams of +1st and 1st orders of these diffracted light beams, are detected. The relative position between the mask and the wafer is adjusted, so that intensities In(+1) and In(-1) of these diffracted light beams are equal to each other. Thus, the mask and the wafer are aligned relative to each other.
However, the diffraction grating of the mask also serves as a reflection type diffraction grating. For this reason, the diffracted light beams of the .+-.1st orders which are diffracted along the path of mask.fwdarw.wafer.fwdarw.mask, interfere with reflected diffracted light beams of the .+-.1st order reflected by the upper surface of the mask. If a distance of a gap between the mask and the wafer is z, the diffracted light beams of the .+-.1st orders have an optical path difference of 2z with respect to the reflected diffracted light beam of the 1st order. If 2z=n.lambda., (.lambda. is a wavelength of incident light beam and n is an integer), the diffracted light beams of the .+-.1st orders are interfered with the reflected diffracted light beam of the .+-.1st order. For this reason, if a gap between the mask and the wafer varies by .lambda./4, the intensity of the .+-.1st order diffracted light beam is considerably decreased, and measurement of the .+-.1st order diffracted light beam is difficult to attain.
Therefore, if the distance of the gap between the mask and the wafer cannot be accurately maintained, the measurement precision of the relative displacement between the mask and the wafer is impaired.
Japanese Patent Disclosure No. 61-116837 describes the following alignment method.
Angle .theta..sub.k of diffraction of the kth order is given by: EQU sin.theta..sub.k =k.multidot..lambda./p (1)
(where p is a pitch of the diffraction gratings) For this reason, if the pitch of the diffraction grating of the mask is p.sub.m and the pitch of the wafer is p.sub.w, 2p.sub.m =p.sub.w is established. The angle of diffraction of diffracted light beam of the 1st order reflected by the diffraction grating of the mask, thus, differs from that of the diffracted light beam of the .+-.1st orders diffracted along the path of mask.fwdarw.wafer.fwdarw.mask.
However, the method described in the disclosure also reported that the intensity of the diffracted light beam of the .+-.1st orders depends on the distance of the gap between a mask and a wafer (High Precision Alignment Method Using Double Diffraction Gratings (No. 5), Seimitsukikai Gakkai Shukitaikai Gakujitsukoen Ronbunshu, 1984, pp. 443-444, NTT Tsuken). If the distance of the gap between the mask and the wafer cannot be maintained within 2 .mu.m, it is difficult to measure the diffracted light beam of the .+-.1st orders. For this reason, the alignment precision between the mask and the wafer is impaired.